sedimentary
Chert (Flint)
Chert (Microcrystalline or cryptocrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale); Color: Gray, brown, or black with light banding; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray, brown, or black with light banding
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, brown, or black with light banding; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of siliceous remains of marine organisms (diatoms, radiolarians) or chemical precipitation from silica-rich groundwater within limestone or chalk beds. Commonly found in Paleozoic to Mesozoic sedimentary strata.
Uses & applications
Historically used for stone tools and starting fires (flint); modernly used as road surfacing, in ceramics, and as decorative gravel. High-quality specimens are used for knapping or as jewelry cabochons.
Geological facts
Flint was arguably the most important resource for prehistoric humans, used to create sharp edges for hunting and tools. When struck against steel, it produces sparks, leading to its name originating from the Old English 'flint' meaning 'hard rock'.
Field identification & locations
Identified in the field by its characteristic smooth, curved (conchoidal) fracture and its ability to scratch glass. Found often as nodules or layers within limestone. Collectors look for sharp edges and unique color banding.
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